Light-induced thermal spin current

Thermal generation of magnonic spin current by the spin Seebeck effect (SSE) and spin-polarized current by the anomalous Nernst effect (ANE) are fascinating phenomena in the field of spin caloritronics. In this paper, we investigate the thermal spin current driven by light in the framework of the SSE and the ANE. Unlike heating by electrical means, the spin current can be enhanced or reduced, and its sign can even be reversed, thanks to the intrinsic characteristic of the penetrating light. Most importantly, by reversing the direction of the incident light, we are able to separate the contributions from the interfacial and bulk temperature gradients to the transverse spin accumulation. Notably, although the overall spin signals are strongly influenced by the frequency of light, the interfacial and bulk spin Seebeck coefficients are intrinsic and frequency independent. Furthermore, with light excitation, we observe a nearly instantaneous steplike response for both the SSE and the ANE, highlighting the importance of spin-charge conversion by spin-orbit coupling for ultrafast spin caloritronics.

Figure 1

$V_{\mathrm{ISHE}}$ of (a) Pt (5 nm)/YIG (0.5 mm)/Pt (5 nm) with top resistor heater, (b) Pt (5 nm)/YIG (0.5 mm)/Pt (5 nm) with bottom resistor heater, and (c) Pt (5 nm)/YIG (0.5 mm)/W (5 nm) with top resistor heater. $V_{\mathrm{ISHE}}$ of Pt (5 nm)/YIG (0.5 mm) by illuminating (d) the top and (e) the bottom.

Figure 2

(a) $V_{\mathrm{ISHE}}$ induced by green laser illuminating top and bottom of Pt (5 nm)/YIG (0.5 mm). (b) Transmittance spectrum for YIG (60 nm, 2.7 μm, 0.5 mm) samples. $V_{\mathrm{ISHE}}$ induced by blue, green, and NIR laser illuminating (c) top of Pt (5 nm)/YIG (60 nm)/GGG (0.5 mm), (d) bottom of Pt (5 nm)/YIG (60 nm)/GGG (0.5 mm), (e) top of Pt (5 nm)/YIG (2.7 μm)/GGG (0.5 mm), and (f) bottom of Pt (5 nm)/YIG (2.7 μm)/GGG (0.5 mm).

Figure 3

Two-dimensional color maps of ${\nabla }_{z}T$ distribution in Pt (5 nm)/YIG (60 nm) with blue light excitation by the comsol simulation for (a) top-light configuration and (b) back-light configuration. The temperature gradient (${\nabla }_{z}T$) excited by the local laser (blue and square) and the uniform heater (red and circle) are plotted as a function of distance from the interface for (c) top-light (top-heater) and (d) back-light (back-heater) configurations. Also shown is the wavelength dependence of the integration ${\int }_{\mathrm{Int}}{\nabla }_{z}Tdx$ along the interface for (e) top-light configuration and (f) back-light configuration. The inset shows the absorption and transmittance spectra for Pt (5 nm) and YIG (60 nm), respectively.

Figure 4

$V_{\mathrm{ANE}}$ of (a) CFB (100 nm), (b) CFB (5 nm), and (c) CFB (3 nm) when illuminating the top and bottom by green light.

Figure 5

The temporal evolution of (a) the $V_{\mathrm{ISHE}}$ of Pt/YIG excited by light (right $y$ axis) and heater (left $y$ axis), and (b) the $V_{\mathrm{ANE}}$ of CFB excited by light with positive saturation magnetization.

Figure 6

Two-dimensional color maps of ${\nabla }_{z}T$ distribution by blue back-light configuration with YIG thickness of (a) 60 nm, (b) 180 nm, (c) 300 nm, (d) 600 nm, (e) 900 nm, and (f) 1200 nm (along the $z$ axis). The interfaces between Pt (5 nm) and YIG ($d$ nm) are indicated by dashed lines.

Figure 7

The YIG-thickness dependence of the integration ${\int }_{\mathrm{int}}{\nabla }_{z}Tdx$ along the Pt/YIG interface with the back-light configuration for (a) rf-sputtered YIG and (b) LPE-YIG. The red solid lines correspond to the best decaying monoexponential fit.

Figure 8

The wavelength dependence of the integration ${\int }_{\mathrm{int}}{\nabla }_{z}Tdx$ along the interface of Pt/LPE-YIG for (a) top-light configuration and (b) back-light configuration. Two-dimensional color maps of ${\nabla }_{z}T$ distribution for top-light configurations with (c) blue, (d) green, and (e) NIR light, and for back-light configurations with (f) blue, (g) green, and (h) NIR light.

Figure 9

The wavelength dependence of (a) the interfacial spin Seebeck coefficient, $S_{\mathrm{Int}}$, (b) the bulk spin Seebeck coefficient, $S_{\mathrm{Bulk}}$, and (c) the total effective spin Seebeck cofficient, $S_{\mathrm{eff}}$, for both sputtered YIG (60 nm) and LPE-YIG (2.7 μm).

Figure 10

Schematic diagram of the SSE induced by (a) resistive heater and (b) light. (c) Schematic diagram of the Seebeck effect induced by light. The temporal evolution of the $V_{\mathrm{ISHE}}$ of Pt/YIG excited by light (right $y$ axis) and resistive heater (left $y$ axis) with magnetic field of (d) 1000 Oe and (e) −1000 Oe. (f) The temporal evolution of Seebeck voltage induced by light.

Figure 11

(a) Schematic diagram of the ANE induced by light. The temporal evolution of the $V_{\mathrm{ANE}}$ of CFB excited by light with magnetic field (b) 900 Oe and (c) −900 Oe.

Figure 12

(a) Schematic diagram of the ANE induced by light with the illumination position at the left side of CFB. The temporal evolution of the voltage excited by light for magnetization orientations of (b) $+y$ and (c) $-y$. (d) Schematic diagram of the ANE induced by light with the illumination position at the right side of CFB. The temporal evolution of the voltage for magnetization orientations of (e) $+y$ and (f) $-y$. (g) Schematic diagram of the ANE induced by light with the illumination position at the middle of CFB. The temporal evolution of the voltage for magnetization orientations of (h) $+y$ and (i) $-y$.